1. Field of the Invention
The present invention relates to a variable-speed control apparatus for controlling the variable speed of an AC motor.
2. Description of the Related Art
Generally, an induction motor is used as an AC motor. The terminal voltage of the induction motor includes a transient voltage represented by the product of the differentiation term of a primary current and the leakage inductance of the induction motor. In controlling the electric current to an AC motor, the transient voltage is reduced by the feedforward compensation method in which the product of the differentiation term of a current command value and a predetermined leakage inductance is computed and the result is added to a voltage command value.
FIG. 1 is a block diagram showing an example of the conventional variable-speed control apparatus for an induction motor which reduces the transient voltage by the feedforward compensation method.
In FIG. 1, an induction motor 3 is operated at a variable speed by converting the AC power provided by an AC power source 1 into an AC power of a desired voltage and frequency by a power converter 2. In the variable-speed control apparatus which controls the power converter 2, the U-phase current actual value i.sub.u and the W-phase current actual value i.sub.w in the 3-phase output of the power converter detected by current detectors 4 (at least two of three phase outputs) are divided by a first coordinate conversion circuit 5 into the M-axis current actual value I.sub.M, which is the current in the M-axis direction (component in the magnetic flux direction) of the motor, and the T-axis current actual value I.sub.T, which is the current in of the T-axis direction (component in the torque direction). The M-axis is perpendicular to the T-axis. A command value generation circuit 7 outputs the M-axis current command value I.sub.M * and T-axis current command value I.sub.T *. An M-axis current adjuster 8 outputs a signal V.sub.M ** which sets the deviation to zero between the M-axis current actual value I.sub.M and the M-axis current command value I.sub.M *. A T-axis current adjuster 9 outputs a signal V.sub.T ** which sets the deviation to zero between the T-axis current actual value I.sub.T and the T-axis current command value I.sub.T *. If these axis current adjuster outputs V.sub.M ** and V.sub.T ** are provided as voltage command values for the power converter 2 through a second coordinate conversion circuit 6, the above described transient voltage is generated.
Then, the leakage inductance set value L* which is predetermined in the command value generation circuit 7, the M-axis current actual value I.sub.M, the T-axis current actual value I.sub.T, the M-axis current command value I.sub.M *, and the T-axis current command value I.sub.T *, are input to a feedforward compensation voltage operation circuit 11 to compute the M-axis feedforward compensation voltage command value V.sub.MF * and the T-axis feedforward compensation voltage command value V.sub.TF *. An M-axis compensation voltage adder 12 adds the M-axis feedforward compensation voltage command value V.sub.MF * and the above described M-axis current adjuster output V.sub.M **, and the sum V.sub.M * is provided as an M-axis voltage command value for the second coordinate conversion circuit 6. Similarly, a T-axis compensation voltage adder 13 adds the T-axis feedforward compensation voltage command value V.sub.TF * and the T-axis current adjuster output V.sub.T **, and the sum V.sub.T * is provided as a T-axis voltage command value for the second coordinate conversion circuit 6. The second coordinate conversion circuit 6 converts the M-axis voltage command value V.sub.M * and T-axis voltage command value V.sub.T * into 3-phase voltage command values v.sub.u *, v.sub.v *, and v.sub.w *. The power converter 2 is controlled using these 3-phase voltage command values v.sub.u *, v.sub.v *, and v.sub.w *.
The operations performed by the above described feedforward compensation voltage operation circuit 11, M-axis compensation voltage adder 12, and T-axis compensation voltage adder 13, are expressed by the following equations (1) and (2). EQU V.sub.M *=V.sub.M **+V.sub.MF *=V.sub.M **+p.multidot.L*.multidot.I.sub.M *(1) EQU V.sub.T *=V.sub.T **+V.sub.TF *=V.sub.T **+p.multidot.L*.multidot.I.sub.T *(2)
where p indicates the differential operator, and L* indicates a leakage inductance set value. PA1 where I.sub.M indicates an M-axis current actual value, I.sub.T indicates a T-axis current actual value, V.sub.M indicates an M-axis voltage actual value, V.sub.T indicates a T-axis voltage actual value, R1* indicates a primary resistance set value, and .omega.1* indicates the frequency set value.
The actual current value and actual voltage value are detected to compute the M-axis induction voltage E.sub.M * and T-axis induction voltage E.sub.T * of an induction motor by the following equations (3) and (4). EQU E.sub.M *=V.sub.M -R1*.multidot.I.sub.M -p.multidot.L*.multidot.*I.sub.M +.omega.1.multidot.L*.multidot.I.sub.T ( 3) EQU E.sub.T *=V.sub.T -R1*.multidot.I.sub.T -p.multidot.L*.multidot.I.sub.T +.omega.1.multidot.L*.multidot.I.sub.M ( 4)
The control by obtaining the induction voltage from the actual voltage value and actual current value is disclosed as, for example, the "speed-sensorless vector control" in the Japanese Patent Publication Tokukohei No. 7-71400.
The M-axis voltage command value V.sub.M *, T-axis voltage command value V.sub.T *, M-axis induction voltage E.sub.M *, and T-axis induction voltage E.sub.T *, are computed by each of the above listed equations. Each of the equations contains an operation for the voltage drop caused by the leakage inductance. Therefore, if there is a difference between the leakage inductance set value L* and the actual leakage inductance value of a motor used in the operation for the voltage drop, then there will be an error in the operation for the induction voltage and in the feedforward compensation by the conventional variable-speed control apparatus described above by referring to FIG. 1. As a result, there will also occur an error in controlling the motor from the feedforward compensation, thereby incurring a disadvantage that, for example, the transient voltage cannot be appropriately reduced.
The present invention aims at providing a circuit for appropriately controlling a motor using a correctly obtained leakage inductance value of the motor.